For decades, the 16th district was the most geographically stable district in Illinois. For more than six decades, in comparison to the other districts in the state, it was almost stationary. While its shape fluctuated slightly after each census, in general it included the northwest corner of the state, extending just far enough to the east to include its largest city, Rockford.[4] By the 1990s, it extended eastward to include part of McHenry County, an outer suburb of Chicago. This geographic stability also contributed to electoral stability. It first became a Rockford-based district for the 1948 election, and from then until 2010 it was represented by just five people, all but one of whom was a Republican.

However, with the new map drawn for 2012, the familiar shape of the 16th was rendered unrecognizable. It was pushed well to the east to take in the extreme exurban region of the Chicago metropolitan area, and stretches from the Wisconsin border to the Indiana border. While it still included most of Rockford's suburbs, half of Rockford itself—essentially the more Democratic portion of the city—was shifted to the 17th district.